The invention includes embodiments that relate to an electrode composition. The invention includes embodiments that relate to an electrode for an energy storage device.
Metal chloride batteries with a molten sodium anode and a beta-alumina solid electrolyte are widely employed for energy storage applications. The energy storage application can include mobile applications, due to their high energy density and long cycle life. To be applicable for mobile applications like hybrid locomotives or plug-in electric vehicles (PHEV), the sodium nickel chloride battery should tolerate power surges (high currents) at both battery charging and discharging, without loss in the working capacity and the cycle life. The sodium nickel chloride batteries are used because of their high theoretical energy density (790 Wh/kg), in addition to their ability to operate over a wide temperature range. The cathode of such a battery is built from nickel metal, sodium chloride NaCl, and a molten secondary electrolyte, NaAlCl4. Nickel is present in excess, and the battery's theoretical capacity is determined by the amount of NaCl. However, the practical energy density after the first full battery charge is much lower than the theoretical value, and the cycle life of such a battery is very short.
The most common way to improve the cell performance is an addition of a small amount of additives to the cathode composition. The use of sodium salts of other halogens (NaF, NaBr and NaI) and elemental sulfur as additives have been tried. However, the above approaches had disadvantages of low working capacity or, in the case of sulfur, non-uniform distribution causing high variability in the battery performance and fast degradation. Addition of iron monosulfide FeS allowed for better sulfur distribution in the electrochemical cell and less variability. However, the above electrodes containing FeS and S in combination with NaX (X=F, Br, I), were not suitable for high current applications because of low working capacity and fast degradation of cells.
Therefore, there exists a need for an improved solution to the long-standing problem of high current cell performance by addition of additives for the electrode that significantly improves the cell working capacity and decreases the capacity degradation rate.